Relatively little is known about the mediators and moderators of effective psychosocial treatment for AUD. Building on the work of a recently completed R21 study, the primary aim of this proposed R01 study is to rigorously test the hypothesized mechanisms of behavior change (MOBC) of motivational interviewing (MI). As in our previous study, MI will be dismantled into three distinct components: 1) full MI (FMI) consisting of its relational (e.g., spirit) and technical (directive) strategies, 2) spirit-only MI (SOMI) consisting of empathic and non-directive counseling, and 3) a self-change control (SCC). A primary goal of the study will be to experimentally manipulate commitment strength, the hypothesized mediator of MI, and demonstrate its relationship to reduced drinking. Problem drinkers diagnosed with AUD (N=300) will be assessed at baseline and randomly assigned to FMI, SOMI, or SCC and followed intensively during treatment and at 1, 3, and 6 months post treatment. Daily process assessment of MI mediators (e.g., commitment strength) will be collected via Interactive Voice Recordings (IVR) and coding of client talk during therapy sessions as part of the assessment procedures. Participants will also provide blood samples at month 1 and 6 to assess for heavy drinking and corroborate self report. We hypothesize that FMI will yield significantly reduced drinking outcomes at end treatment and during the post treatment follow-up period relative to the other conditions;that FMI will significantly increase commitment strength relative to the other conditions;and that increased commitment strength will mediate the relationship of condition effects on drinking outcomes. In addition, as a secondary aim we hypothesize that baseline client motivation (assessed using IVR) will significantly moderate the relationship between treatment condition and outcome. Treatment effects (FMI relative to SOMI/SCC) will be stronger for those with low motivation. A number of additional secondary aims and exploratory analyses are described to elucidate the MOBC of MI beyond those hypothesized as primary aims. PUBLIC HEALTH RELEVANCE: Motivational Interviewing (MI) is an effective intervention with tremendous implications for a variety of populations suffering from conditions and disorders related to public health, including smoking, gambling, exercise adherence, nutrition, illicit drug use, and alcohol abuse. This R01 builds on a previously implemented pilot study to investigate the mechanisms of change of MI. If its aims are achieved, it will strengthen the efficacy of MI, as well as AUD treatments in general, which can be utilized and implemented in a variety of health settings.